#include<iostream>
#include<cmath>
using namespace std;

int main()
{
	double a, b, c;
	cin >> a >> b >> c;
	double n = sqrt((a + b + c) / 2 * ((a + b + c) / 2 - a) * ((a + b + c) / 2 - b) * ((a + b + c) / 2 - c));
	printf("%.1f", n);
	
	return 0;
}
